Actuator with inherent position sensor

ABSTRACT

An actuation system is proposed for an optical system, comprising a voice coil motor for actuating the optical system, the voice coil motor comprising a magnet and an electric coil, a position measuring unit for measuring the position of the electric coil and providing a position feedback signal, and a control unit for closed loop control of the position of the optical system based on a target position and the position feedback signal, used for generating a drive signal for the electric coil. According to the disclosure, a ferromagnetic element is arranged in proximity to the electric coil so that the inductance of the electric coil depends on its position. Further, the position-measuring unit measures the inductance of the electric coil and determines the position of the electric coil based on the determined inductance.

TECHNICAL FIELD

The present document relates to an actuation system and method forcontrolling a motor, e.g. a voice coil motor (VCM), in an opticalsystem.

BACKGROUND

Voice coil motor (VCM) actuators creating a one-directional(single-axis) stroke can be used for camera autofocus (AF), multi-axisoptical image stabilization (OIS) or multi-axis haptic feedbackapplications. In case of multi-axis applications, a respective number ofVCM actuators can be arranged accordingly.

Current VCM systems with higher performance employ Hall sensors for theposition feedback of the moving part, which includes, e.g. coil and lensbarrel in case of an AF camera module. This construction has someadvantages, such as low current in idle stage as well as fast AFresponse. However, this setup requires additional elements, such aspermanent magnets and Hall sensor ICs.

An alternative way is to use a spring-preloaded VCM with an excursion,which approximately depends on a linear restoring force that allows,after calibration, linear control via a current source. In general, thissetup is used for more economic solutions. The drawback of this solutionresides in increased power consumption as well as limited AF speed,which is limited by a resonance frequency of the mass-spring system.

SUMMARY

Therefore, there is a need for a more robust and simplified assembly forVCM implementation with reduced system costs. A reduced number ofcomponents further reduces system size.

In view of this need, the present disclosure proposes an actuationsystem and a corresponding method having the features of the respectiveindependent claims for controlling a motor, e.g. a voice coil motor(VCM) in an optical system.

According to a broad aspect of the disclosure, an actuation system isprovided for an optical system. The actuation system comprises a voicecoil motor (VCM) for actuating the optical system. In general, the voicecoil motor comprises at least one magnet (e.g. a permanent magnet) andan electric coil (e.g. an inductor). In particular, the electric coilmay be movable together with one or more other elements of the opticalsystem, for example, a lens of an image-focusing module. The actuationsystem further comprises a position-measuring unit for measuring therelative position of the moving part of the VCM system, e.g. theelectric coil, versus the stationary part, e.g., the housing. Theposition-measuring unit may also be used for providing a positionfeedback signal. Based on the measured position of the electric coil,the position-measuring unit may provide the position feedback signal toactively control the position of the optical system.

According to the disclosure, the actuation system also comprises acontrol unit for closed loop control of the position of the opticalsystem. More specifically, the control unit may receive the positionfeedback signal and control the position of the optical system based ona target position and the position feedback signal. Further, the controlunit may be used for generating a drive signal for the electric coil.Accordingly, the position of the electric coil may be changed throughthe applied drive signal, which may, for example, generate an axialforce.

Moreover, the actuation system may comprise a ferromagnetic element. Theferromagnetic element may be arranged in proximity to the electric coilso that the inductance of the electric coil varies depending on itsrelative position within the optical system, in particular relative tothe ferromagnetic element. In particular, the inductance of the electriccoil may vary based on a relative position between the electric coil andthe ferromagnetic element. Furthermore, the position-measuring unit maymeasure the variable inductance of the electric coil. It is noted thatthe electric coil may be discharged prior to the measurement.Subsequently, the position-measuring unit may determine the position ofthe electric coil based on the determined inductance. In someembodiments, the inductance of the electric coil may be determined basedon a measured voltage and/or a measured current that allows determiningof the coil inductance based on the measured value.

Since the inductance of the electric coil depends on the relativeposition between the electric coil and the ferromagnetic element, theposition of the electric coil can be determined through the knowledge ofits relative position by measuring the variable inductance of theelectric coil. In other words, the position feedback signal can bedetermined from the measured coil inductance, e.g. with the use of amapping function or a lookup table. In addition, interpolation betweendata points may be used to determine the coil position more precisely.

In some embodiments, the actuation system may further comprise asummation unit for adding an AC signal to the drive signal for measuringthe inductance of the electric coil. In particular, the AC signal mayhave a relatively higher frequency than the frequency of the drivesignal (e.g. DC in steady state) for the electric coil. In other words,the drive signal for the electric coil, which causes the movement (i.e.a position change) of the electric coil, has a relatively lowerfrequency than the AC signal. The summed-up signal comprising the drivesignal and the AC signal is then supplied to the coil.

As a result, by measuring the inductance of the electric coil forposition sensing, the complexity of a VCM system can be reduced.Besides, this sensing technique makes any additional elements (e.g.magnets) for position sensing dispensable. For example, no Hall sensorsare needed for this sensing technique. It is therefore appreciated thatthis position sensing technique using the knowledge of coil inductancecan provide similar performance with a simple setup.

In some embodiments, the voice coil motor may further comprise anotherelectric coil. The two coils may be arranged in series and both may movetogether in the moving part of the VCM. In particular, theposition-measuring unit may measure a differential inductance betweenthe electric coil and another electric coil. One of the electric coilsmay serve as a reference coil. Optionally, both coils may be measuringcoils, one having a reversed direction of windings. In general, bothcoils may be driving coils and coils for differential inductancemeasurement.

The position-measuring unit may determine a relative position betweenthe electric coil(s) and the ferromagnetic element based on thedetermined differential inductance. An example of the position-measuringunit may comprise a Maxwell bridge circuitry. Other type of circuitrymay also be used as/for the position measuring unit. In someembodiments, a single coil inductance measurement may be conducted usingthe electric coil and another electric coil. In this case, one of theelectric coils may comprise a Gyrator, e.g. the reference coilequivalent circuitry may comprise a Gyrator.

As a result, by measuring the coil inductance (e.g. the single/absoluteinductance and/or the differential inductance), one can use a VCMactuator itself for position sensing without applying Hall sensors forposition feedback, which also reduces mechanical/electrical connectionsbetween the modules thereof. In other words, the proposed system employsa simplified and efficient position sensing technique to replace Hallsensors by using the existing parts of the VCM itself for positionsensing. It is appreciated that the proposed system has similarperformance but can be implemented with a more cost-effective setup.

According to another broad aspect of the disclosure, an actuation systemis provided for an optical system. The actuation system comprises avoice coil motor (VCM) for actuating the optical system. In general, thevoice coil motor comprises at least one magnet (e.g. a permanent magnet)and an electric coil (e.g. an inductor). In particular, the electriccoil may be movable together with one or more other elements of theoptical system, for example, a lens of an image-focusing module. Theactuation system further comprises a position-measuring unit formeasuring the position of the electric coil. The position-measuring unitmay also be used for providing a position feedback signal. Based on themeasured position of the electric coil, the position-measuring unit mayprovide the position feedback signal to actively control the position ofthe optical system.

According to the disclosure, the actuation system also comprises acontrol unit for closed loop control of the position of the opticalsystem. More specifically, the control unit may receive the positionfeedback signal and regulate the position of the optical system based ona target position and the position feedback signal. Further, the controlunit may be used for generating a drive signal for the electric coil.Accordingly, the position of the electric coil may be changed throughthe applied drive signal, which can, for example, generate an axialforce.

Moreover, the actuation system may comprise an excitation unit. Theexcitation unit may be used for generating a higher frequency excitationsignal. The excitation signal may be a periodic alternating currentsignal with alternating portions of positive and negative currentsections. In particular, the high frequency excitation AC signal may besuperimposed on the drive signal (e.g. through a summation unit) so thatthe coil current comprises a first component caused by the drive signaland a second component caused by the excitation signal. Further, thehigh frequency excitation signal (second coil current component) maycause a designated Back-EMF (back electromotive force) component of theelectric coil. More specifically, the position-measuring unit maymeasure the Back-EMF component of the coil caused by the AC excitationsignal. Subsequently, the position-measuring unit may determine theposition of the electric coil based on the measured Back-EMF component.It is noted that the electric coil may be discharged prior to themeasurement of the Back-EMF of the coil. Typically, the Back-EMF issmall compared to the excitation AC. Sensing under applied AC excitation(and possibly driving current) means sensing a small phase shift, whichis more difficult. In embodiments, the AC excitation current may bedischarged together with the driving current component as both aredriven via the same terminals (thereby the coil can be discharged).

In some embodiments, the voice coil motor of the actuation systemcomprises a permanent magnet. The permanent magnet may cause a staticpermanent magnetic field. In particular, the Back-EMF of the coil mayvary based on a relative position (e.g. an overlap portion) between theelectric coil and the static permanent magnetic field. The Back-EMFfurther depends on the movement (speed) of the coil, which is modulatedwith the higher frequency excitation signal. It is noted that theexcitation current of the drive signal for the electric coil mayperiodically reach zero. According to this, the position-measuring unitmay measure a difference in the Back-EMF of the coil at subsequentcurrent zero crossings of the excitation signal for the electric coil toobtain the overlap portion for the position determination from anevaluation of the Back-EMF difference.

Since the Back-EMF of the electric coil depends on the overlap portionbetween the electric coil and the static permanent magnetic field, theposition of the electric coil can be determined through the knowledge ofthe overlap portion by measuring the Back-EMF of the electric coil. Inother words, the position feedback signal can be determined from themeasured Back-EMF, e.g. with the use of a mapping function or a lookuptable. In addition, interpolation between data points may be used todetermine the coil position.

According to the disclosure, the position measuring unit may measure theBack-EMF based on a difference in the Back-EMF at subsequent currentzero crossings of the excitation signal. The position-measuring unit mayalso measure a Back-EMF offset. According to some embodiments, aresidual coil speed may be further obtained based on the Back-EMF offsetand/or the difference in the Back-EMF. The position-measuring unit mayfurther measure the Back-EMF (e.g. the absolute Back-EMF) based on theoverlap portion. Accordingly, the position feedback signal may compriseinformation on the determined position (through the knowledge of theoverlap portion) and the residual coil speed (that causes an offset tothe Back-EMF waveform) that might be preferably evaluated close to or attime instants of zero crossing of the time varying excitation signal.Further, the actuation system may also comprise a calibrating unit toobtain linear positioning.

As a result, by using the measured Back-EMF of the electric coil asinformation to determine the overlap between the coil and the staticpermanent magnetic field for position sensing, no Hall sensors forposition feedback are required, which simplifies a VCM setup with fewermodules and connections between/in the modules thereof. In other words,the proposed system uses the existing parts of the VCM itself to replaceHall sensors for position sensing. Thus, a good response time for a VCMsystem can be obtained by using this cost-effective way.

According to another aspect, a method for actuating a voice coil motorin an optical system is provided. In general, the voice coil motor has amagnet and an electric coil. As mentioned above, the electric coil maybe movable together with one or more other elements of the opticalsystem, for example, a lens of an image-focusing module.

According to the disclosure, the method comprises measuring theinductance of the electric coil. The method also comprises determiningthe position of the electric coil. In detail, the position of theelectric coil may be determined based on the measured inductance.Furthermore, the method comprises providing a position feedback signal.In particular, the position feedback signal may be provided based on thedetermined position. Subsequently, the method comprises controlling theposition of the optical system. The position of the optical system maybe regulated based on a target position and the position feedbacksignal. Also, the method comprises generating a drive signal for theelectric coil based on the determined position. Accordingly, theposition of the electric coil may be changed through the applied drivesignal, which may, for example, generate an axial force.

In detail, the inductance of the electric coil may depend on itsposition. More specifically, the inductance of the electric coil mayvary based on a relative position (e.g. an overlap portion) between theelectric coil and a ferromagnetic element. In some embodiments, themethod may further comprise measuring a voltage and/or a currentassociated with the electric coil. Thus, the inductance of the electriccoil may be determined based on the measured voltage and/or the measuredcurrent. Also, the method may further comprise discharging the electriccoil prior to the measuring.

Furthermore, the drive signal for the electric coil may have a lowfrequency. The method may further comprise adding an AC signal to thedrive signal for measuring the inductance of the electric coil. It isnoted that the AC signal may have a relatively higher frequency than thefrequency of the drive signal for the electric coil. As the drive signalfor the electric coil, which causes the movement (i.e. a positionchange) of the electric coil, has a relatively lower frequency than theAC signal, the electric coil may be quickly discharged so as to speed upthe measurement of the electric coil inductance.

As a result, by measuring the inductance of the electric coil forposition sensing, the total response time for a VCM system can bereduced. Besides, this sensing method makes any additional magnets forposition sensing dispensable. For example, no Hall sensors are neededfor this sensing method. Therefore, the proposed position sensing methodusing the knowledge of coil inductance can provide higher performancefor a VCM system in a cost-effective way.

In some embodiments, the voice coil motor may further comprise anotherelectric coil. In these cases, the method may further comprise measuringa differential inductance between the electric coil and another electriccoil. Further, the method may also comprise determining a relativeposition between the electric coil and the ferromagnetic element basedon the determined differential inductance. As a result, the proposedmethod using a VCM actuator itself for position sensing can provide fora VCM system without applying any Hall sensors for position feedback,which also reduces connections between/in the modules thereof. In otherwords, the proposed method enables a simplified setup for efficientposition sensing, thereby providing higher performance in acost-effective way.

According to another aspect, a method for actuating a voice coil motorin an optical system is provided. As mentioned above, the voice coilmotor has a magnet and an electric coil. In particular, the electriccoil may be movable together with one or more other elements of theoptical system, for example, a lens of an image-focusing module.

According to the disclosure, the method comprises generating a drivesignal for the electric coil. The method also comprises generating ahigh frequency excitation signal. In particular, the higher frequencyexcitation signal may be superimposed on the drive signal for the coilso that the coil current has a DC component caused by the drive signaland an AC component caused by the excitation signal.

Further, the higher frequency excitation signal may cause a varyingBack-EMF component and the DC coil current may cause a Back-EMF offsetof the electric coil. The method comprises measuring the variableBack-EMF of the coil caused by the excitation signal. Subsequently, themethod further comprises determining the position of the electric coilbased on the measured Back-EMF. It is noted that the method may furthercomprise discharging the electric coil prior to the measuring of theBack-EMF of the coil.

According to the disclosure, the method also comprises providing aposition feedback signal. In particular, the position feedback signalmay be provided based on the determined position as disclosed above. Themethod further comprises a closed loop controlling the position of theoptical system. More specifically, the position may be regulated basedon a target position and the position feedback signal.

In some embodiments, the voice coil motor of the actuation system mayfurther comprise a permanent magnet. Thus, the Back-EMF of the coil mayvary based on a relative position (e.g. an overlap portion) between theelectric coil and a static permanent magnetic field caused by thepermanent magnet of the voice coil motor. It is also noted that acurrent of the drive signal for the electric coil may reach zero whenthe electric coil is discharged. More specifically, the driving signalmay comprise a DC current part (component) and an AC current part(component), and the AC current part may reach zero value at certaininstances when the alternating current changes polarity. According tothis, the method may further comprise measuring a difference in theBack-EMF of the coil at subsequent current zero crossings of the driveor excitation signal for the electric coil to obtain the overlap portionfor the position determination. The overlap portion may be determinedbased on the measured difference in the Back-EMF. In some embodiments,the method may further comprise measuring a Back-EMF offset. The methodmay also comprise obtaining a residual coil speed based on the Back-EMFoffset and/or the difference in the Back-EMF. The method may alsocomprise measuring the Back-EMF (e.g. the absolute Back-EMF) based onthe overlap portion. Accordingly, the position feedback signal maycomprise knowledge of the determined position (through the knowledge ofthe overlap portion) and the residual coil speed.

As a result, by measuring the Back-EMF of the electric coil to obtainthe overlap between the electric coil and the static permanent magneticfield and/or the residual coil speed for position sensing, no Hallsensors for position feedback are required, which simplifies a VCM setupwith fewer connections between the modules thereof. In other words, theproposed method uses the existing parts of the VCM itself for positionsensing to replace Hall sensors. Also, a good response time for a VCMsystem can be obtained in a cost-effective way.

It should be noted that the methods and systems including its preferredembodiments as outlined in the present document may be used stand-aloneor in combination with the other methods and systems disclosed in thisdocument. In addition, the features outlined in the context of a systemare also applicable to a corresponding method. Furthermore, all aspectsof the methods and systems outlined in the present document may bearbitrarily combined. In particular, the features of the claims may becombined with one another in an arbitrary manner.

In the present document, the terms “couple”, “coupled”, “connect”, and“connected” refer to elements being in electrical communication witheach other, whether directly connected e.g., via wires, or in some othermanner.

BRIEF DESCRIPTION OF THE DRAWINGS

The application is explained below in an exemplary manner with referenceto the accompanying drawings, wherein:

FIG. 1(A) schematically illustrates an example of an actuation systemusing a voice coil motor (VCM) for general purpose;

FIG. 1(B) schematically illustrates an example of an actuation systemusing a voice coil motor (VCM) for a camera module;

FIG. 2(A) schematically illustrates an arrangement of a spring-preloadedVCM actuator;

FIG. 2(B) schematically illustrates an arrangement of a VCM actuatorwith a Hall sensor;

FIG. 2(C) graphically illustrates a speed-time diagram of a VCM actuatorwithout spring restoring force;

FIG. 3(A) schematically illustrates an arrangement of an actuationsystem for an optical system with position sensing according toembodiments of the disclosure;

FIG. 3(B) schematically illustrates exemplary components of an actuationsystem;

FIG. 4 schematically illustrates a flow diagram of an example method foroperating the actuation system of FIG. 3 according to embodiments of thedisclosure;

FIG. 5(A) schematically illustrates an arrangement of an actuationsystem for an optical system with position sensing according toembodiments of the disclosure;

FIG. 5(B) schematically illustrates a measurement circuitry fordifferential-position sensing as illustrated in the actuation system ofFIG. 5(A) according to embodiments of the disclosure;

FIG. 5(C) schematically illustrates a measurement setup fordifferential-position sensing as illustrated in the actuation system ofFIG. 5A according to embodiments of the disclosure;

FIG. 5(D) schematically illustrates an example circuitry of a Gyrator;

FIG. 6(A) schematically illustrates an arrangement of an actuationsystem for an optical system with position sensing according toembodiments of the disclosure;

FIG. 6(B) schematically illustrates an equivalent circuit for positionsensing as illustrated in the actuation system of FIG. 6(A) according toembodiments of the disclosure;

FIG. 7 schematically illustrates a flow diagram of an example method foroperating the actuation system of FIG. 6(A) according to embodiments ofthe disclosure;

FIG. 8 graphically illustrates variation of the Back-EMF and the coilcurrent over time according to embodiments of the disclosure;

FIG. 9 schematically illustrates an actuation system with inherentposition sensor according to embodiments of the disclosure;

FIG. 10 schematically illustrates an arrangement of a linear-motor-likeactuation system with position sensing via a VCM with variablecoil-magnet overlap according to embodiments of the disclosure.

DETAILED DESCRIPTION

FIG. 1(A) schematically illustrates an example of an actuation systemusing a voice coil motor (VCM) for general purpose. The construction fora general-purpose VCM actuator without spring restoring force comprisesa housing 101 which may be made of, e.g. ferromagnetic material toprovide a magnetic flux path. The VCM actuator also comprises apermanent magnet 102 and a coil 103. The coil 103 is mounted on asuspension link (e.g. through a coil cradle). With air gaps between thecoil 103 and the housing 101 as well as between the coil 103 and thepermanent magnet 102, the coil 103 is moveable between the housing 101and the permanent magnet 102. The housing 101 may have a center pin toclose the magnetic flux path and to provide sliding support for asuspension link 107 having a coil cradle for carrying the coil 103. Themoving direction of the suspension link 107 is shown by the doublearrow.

For example, in case of a camera module (i.e. a VCM camera module) asshown in FIG. 1(B), the construction is adapted to allow an arrangementof a coil mounted on a lens barrel, including the housing 101, thepermanent magnet 102, the coil 103, a lens barrel 104, a lens 106 and animage sensor 105. In general, the coil 103 together with the permanentmagnet 102 form the VCM, and the lens barrel 104 typically occupies thecenter of the VCM.

VCMs of different constructions can include a spring to apply arestoring force to the moving mass (e.g. to reach a predefined idlestate per default). FIG. 2(A) schematically illustrates an arrangementof a spring-preloaded VCM actuator. The spring-preloaded VCM actuatorincludes a stationary permanent magnet 202, a coil 203 and a spring 206.The coil 203 is mounted with the spring 206 and is moveable along thedirection as indicated by the arrow 204. The coil terminals are markedwith A, B. As in the case of a camera module VCM, this springfunctionality can be realized together with leads for supplying currentto the moveable coil.

To improve the performance of a VCM actuation system, a Hall sensor canbe employed for the position feedback of the moving part (i.e. the coiltogether with the lens barrel). FIG. 2(B) schematically illustrates anarrangement of a VCM actuator with a Hall sensor (stationary). Inaddition to the permanent magnet 202 and the coil 203 as shown in FIG.2(A), the VCM actuator further includes a magnet 207 fixed to the movingcoil 203 and a Hall sensor 208 instead of the spring 206. The magnet 207is fixed to the coil 203 and is moveable together with the coil 203along the direction as indicated by the arrow 204. Advantages of thisconstruction are low current in the idle stage as well as fast autofocus(AF) response, but this kind of VCM actuator requires additionalelements such as magnets and Hall sensor ICs.

Generation of force for moving the actuator can be realized by applyingcurrent to the coil in general, permanent magnets are used to generate amagnetic flux perpendicular to the windings of the coil. The generatedelectromagnetic force (F) depends on the coil current (i_(coil)) and isbasically given by F=B×i_(coil)×l, where l represents the total lengthof coil wire perpendicular to the magnetic field B. Thus, the motorforce is a function of the magnetic flux density of the coil, thecurrent and the number of windings. Besides, the permanent magnet hasthe permeability of air (μ_(r)=1), which means that the VCM coil doesnot change its inductance when it changes its position relative to thepermanent magnet. In other words, the permanent magnet is neutral forthe inductor/inductance when the VCM coil is moving in the magnet fieldof the magnet. As such, the inductor/inductance will not change itsvalue during movement in the magnetic field.

Furthermore, the impedance of the coil may be dominated by theresistance thereof within the VCM's operating bandwidth. That is, thecoil inductance may depend on the effective magnetic path length and maybecome apparent at higher frequencies. In practice, VCM or linear-motorlike actuators for camera autofocus (AF) or multi-axis optical imagestabilization (OIS) are likely designed with the target to keep theweight of the moving part as small as possible. As such, the coil of aVCM or a linear motor may be referred to as the moving part. However, insome cases, the coil is bonded to e.g. the housing of the actuator (andthereby is regarded as stationary), whereas other elements (such aspermanent magnets) may contribute to the moveable mass accordingly.

VCM actuators are used for different applications that require assertionof controlled force over a limited stroke. The model of such a system(i.e. a VCM actuator without spring restoring force) comprises a masswhich includes the coil and the lens barrel and which may be excited bya controlled force (i.e. electromotive force (EMF) via the coil currentand a statically-applied magnetic field) and a limited disturbance inform of an uncontrollable force (e.g. impact of gravitation depending oncamera orientation, acceleration of the camera itself and staticfriction). Also, the movement of the mass may be damped by the airenclosed in the module and sliding friction. In general, the impact ofthe coil terminals and affiliated connections can be expected to be low.For the case of VCM actuators without a spring restoring force, theinitial position of the actuator may be undefined and depend on movementhistory.

For the case of camera autofocus (AF) actuators, a new lens positionshould be reached within 10 ms to meet the most general performancerequirement. A typical profile of speed to stroke is shown in FIG. 2(C).The top part of FIG. 2(C) graphically illustrates a speed-time diagramof a VCM actuator without spring restoring force. The bottom part ofthis figure shows the corresponding force. Based on the change of thespeed, the diagram can be divided into three consecutive areas, i.e.high acceleration area 209, high retardation area 210 and fine-searcharea 211. The motor starts with high (maximum) acceleration and movesclose to the middle of the targeted change in distance (i.e. the traveldistance) as shown by the high acceleration area 209. When the positionreaches close to the middle of the target position (or half the traveldistance), the motor has to break with the maximum speed and the forcechanges to high (maximum) retardation (of ideally the same maximumforce) as shown by the retardation area 210. When the position reachesclose to the target position, a regulated fine-search movement isapplied to ensure the required positioning accuracy, as shown by thefine-search area 211. It is noted that the fine-search area 211 shouldbe as small as possible. In an ideal way, the reverse point (from theacceleration to the breaking) should be half of the travel distance, ifthe acceleration and the breaking processes have the same behavior.Accordingly, the regulation part (i.e. the fine-search moving) will endat the target position.

A general requirement for high performance camera modules is that thepoint-to-point movement of the actuator should be completed within about10 ms. Thus, the position detection needs to be much faster to detectthe optimum point for application of the reverse force (i.e. to measurethe reverse point). Due to mechanical limitations, the bandwidth of aVCM actuator does not exceed 1 kHz, and the bandwidth of the positionmeasurement should be at least a factor of 20 higher than the bandwidthof the VCM (i.e. 20 kHz or higher). For a total AF lens operating rangeof 2 mm, the maximum speed of the moving part may be about 2 mm/10ms*2=0.4 m/s. Of course, the maximum speed of the VCM moveable part, aswell as the realizable operating range, depends on the constructionaldetails.

FIG. 3(A) schematically illustrates an arrangement of an actuationsystem for an optical system with position sensing according toembodiments of the disclosure. The actuation system 300 comprises avoice coil motor (VCM) for actuating the optical system and may bereferred as a VCM actuator. The voice coil motor includes at least onemagnet 302 (which can be, e.g. a stationary permanent magnet) and anelectric coil 303. Based on the current applied to the coil 303, thecoil moves in the magnetic field of the magnet 302 along the directionas indicated by the arrow 304. The actuation system 300 also includes aposition-measuring unit (not shown in FIG. 3(A)) for measuring theposition of the electric coil and providing a position feedback signal.Moreover, a ferromagnetic element 306 (e.g. a stationary small ironplate, shown only schematically) is arranged in proximity to theelectric coil 303, so that the inductance of the electric coil 303depends on its position. In particular, the ferromagnetic element 306may have one or more iron parts, and the magnet(s) 302 for the motor VCMcan be placed between the irons. As mentioned above, the movingdirection is shown by arrow 304. The coil during the movement will bemore or less coved by the iron (i.e. the ferromagnetic element 306) andwill change the value of its inductance depending on the overlap betweeniron and coil. This change of the coil inductance thus indicates theposition of the moving inductor (i.e. the coil 303).

To provide the position feedback signal, the position-measuring unitmeasures the inductance of the electric coil 303 and determines theposition of the electric coil 303 based on the determined inductance. Ingeneral, the inductance of the electric coil may be measured/determinedbased on a measured voltage and/or a measured current. The positionfeedback signal thus contains information of the position of the coil303. The actuation system 300 further includes a control unit (not shownin FIG. 3(A)) for closed loop control of the position of the opticalsystem based on a target position and the position feedback signal.Also, the control unit can be used for generating a drive signal for theelectric coil 303. The coil 303 is supplied with drive current for itsmovement by the drive signal. Thus, position sensing can be achieved viathe position-depending modulation of VCM coil inductance.

It is noted that the embodiment shown in FIG. 3(A) comprises a singleend inductor, which provides a position-sensing signal that may be anonlinear function of the coil position. Also, the VCM actuatordescribed herein is not spring preloaded and therefore is operatingwithout a spring restoring force.

However, in another embodiment, more than one inductor may be employedfor the VCM actuator and/or the VCM actuator may include a spring thatdoes not significantly affect the dynamics of the system for at leastone of the intended actuator operating modes. It is further noted thatthe resonant frequency of the resulting spring mass system is generallysignificantly smaller than the required control bandwidth.

According to the embodiment of FIG. 3(A), the coil of the VCM actuatorcomprises an inductor having its inductance changing depending on therelative position between the coil and the additional ferromagneticmaterial. The change of the coil inductance is established by theferromagnetic material in proximity of the coil (and thereby in the pathof the magnetic field lines). In general, this inductance change isestablished by changing the relative position, e.g. overlap, between themovable coil (windings) and the ferromagnetic material.

In detail, one part of the magnetic path length of the inductor (coil)is in air (μ_(r)=1) and the other part is inside the range offerromagnetic material (μ_(r)>>1). The effective magnetic path length(normalized to air) can then be represented as:I _(e) =I _(g) +I _(m) /μr.fe,where I_(g) denotes the path length in air (μ_(r)=1) and I_(m) denotesthe path in the range of ferromagnetic material (μ_(r,fe)>>1). The coilinductance can then be written asL=μ ₀×μ_(r) ×N ² ×A/I _(e),where N represents number of turns of the coil windings and A representsthe cross-section area of the coil windings. Thus, by arranging someiron/ferrite material near by the inductor, the inductorvalue/inductance can be modified in dependence on the relative positionbetween coil and ferrite material. The additional permanent magnets areused for generating the driving force of the actuator. The resultingposition-dependent coil inductance may (based on positioning of themagnetic field lines) have a nonlinear dependency. In the embodiment inFIG. 3(A), the inductance may be expected to change around ˜5% over thecoil moving range.

In embodiments, there may exist a design target to generate anelectromagnetic force proportional to coil current over the wholeworking range. In order to achieve this target, the same number of coilturns should remain exposed to the magnetic field over the whole workingrange of the VCM. This objective may not be compromised by changing thecoil inductance depending on the displacement of the coil from theposition of the permanent magnet. Furthermore, the movement of a coilthat carries current in the vicinity of ferromagnetic material mayprovoke a breaking force. This force can exist when the magnetic fluxcomponent is perpendicular to the ferromagnetic material and may beconsidered in system dynamics.

Also, the inductance change with the movement of the current driven coil(due to the change in relative position between the coil andferromagnetic material) may impact the voltage via the inductor as wellas the required driving voltage capability of a current source. Also,the magnetic field of the permanent magnet and that caused by the coilcarrying current are perpendicular to each other at the ferromagneticstationary material, both of which can impact the magnetic flux (lines)within the ferromagnetic material. It can therefore be expected that thepermanent magnetic field reduces the effective permeability of theferromagnetic material with view on the coil inductance.

As mentioned above, the mechanical resonant frequency of the mass systemin a VCM is generally smaller than the required control bandwidth. Thus,an AC signal may be overlaid in the measurement system. The actuationsystem may further comprise a summation unit (not shown) for adding theAC signal to the drive signal for measuring the inductance of theelectric coil. In particular, the AC signal has a relatively higherfrequency than the frequency of the drive signal for the electric coil.Accordingly, the coil driving current to generate an axial force hasharmonics of relatively lower frequency, while the coil AC currentoverlaid in the measurement system to assess (measure) the resultinginductance is of relatively higher frequency. The coil inductance mayalso be discharged (e.g. to zero current) prior to the measurement.Further, any residual speed from an ongoing coil movement may introducea DC offset voltage via the coil terminals. A solution to this effectmay be to decouple the evaluated AC voltage of the coil from the DCoffset.

FIG. 3(B) schematically illustrates exemplary components of theactuation system 300. The actuation system 300 may comprise aposition-measuring unit 371 for measuring a position of an electric coil374 and providing a position feedback signal. The actuation system 300may comprise a control unit 372 for closed loop control of a positionbased on a target position and the position feedback signal, and forgenerating a drive signal for the electric coil 374. The actuationsystem 300 may comprise an excitation unit 373 for generating a highfrequency excitation signal. The actuation system 300 may comprise asummation unit 376 for adding the high frequency excitation signal tothe drive signal. Further, the actuation system 300 may comprise acalibrating unit 375 to obtain linear positioning and/or to compensatefor tolerances.

FIG. 4 schematically illustrates a flow diagram of an example method 400for operating the actuation system 300 according to embodiments of thedisclosure. The method 400 comprises measuring (step 401) the inductanceof the electric coil. The method 400 also comprises determining (step402) the position of the electric coil based on the measured inductance.The method further comprises providing (step 403) a position feedbacksignal based on the determined position. The method comprisescontrolling (step 404) the position of the electric coil (and therebythe optical system) based on a target position and the position feedbacksignal. The control method may be a closed-loop control. The method alsocomprises generating (step 405) a drive signal for the electric coil.

FIG. 5(A) schematically illustrates an arrangement of an actuationsystem for an optical system with position sensing according toembodiments of the disclosure. The actuation system 500 comprises avoice coil motor (VCM) for actuating the optical system and may bereferred as a VCM actuator. The voice coil motor includes at least onemagnet 502 (which can be, e.g. a permanent magnet) and an electric coil503. According to the embodiment of FIG. 5(A), the windings of theelectric coil 503 may be separated in two sections 503-1, 503-2 whichprovide a center tap C and two winding terminals A, B. For the purposeof actuating the VCM (via generation of the axial force), the coils canbe operated as one single VCM coil 503. For example, by supplying thedriving current via terminals A and B. However, for the purpose ofposition measuring, this construction can be regarded as using twoinductors. If two different winding directions are used, then one of thewindings may be used for generating the EMF. Due to the current appliedto the coil 503, the coil can move in the magnetic field of the magnet502 along the direction as indicated by the arrow 504. Similar to theactuation system 300, the actuation system 500 also includes aposition-measuring unit (not shown in FIG. 5(A)) for measuring theposition of the electric coil and providing a position feedback signal.Moreover, a stationary ferromagnetic element 506 (e.g. an iron plate) isarranged in proximity to the electric coil 503, so that the inductanceof the electric coil 503 depends on its position. In particular, theferromagnetic element 506 may have one or more iron parts, and themagnet(s) 502 for the motor VCM can be placed between the irons. Asmentioned above, the moving direction is shown by arrow 504. During themovement, the electric coil sections will be more or less coved by theiron (i.e. the ferromagnetic element 506) and will change the value ofits inductance. This change of the coil sections inductances thusindicates the position of the moving inductor (i.e. the coil 503).

In detail, the center tap C allows differential measurement of therelative position (i.e. between the respective coils and theferromagnetic material), which depends on the inductance related to theindividual sections. For example, during the movement, the inductance ofthe partial coil 503-1 is changed from L_(o) to L_(o)+ΔL, while theinductance of the partial coil 503-2 is changed from L_(o) to L_(o)−ΔL,thereby keeping the overall inductance of the coil 503 constant (2L_(o))during the moving process. With the knowledge of the coil inductancechange (ΔL), the position of the coil 503 can then be determined.

There are several ways to measure the inductor (the inductance) for theposition detection. FIG. 5(B) schematically illustrates a measurementcircuitry for differential-position sensing as illustrated in theactuation system 500 according to embodiments of the disclosure.According to the embodiment, the position-measuring unit comprises aMaxwell bridge (Maxwell-Wien bridge) circuitry. However, in otherembodiments, the measurement may be also based on other possiblecircuitry or without any compensation. FIG. 5(B) depicts a resultingequivalent network for evaluation of the differential inductance using aMaxwell bridge circuitry. The inductor L1 (together with the resistorR1) may be regarded as equivalent to the partial coil 503-1, and theinductor L2 (together with the resistor R2) may be regarded asequivalent to the partial coil 503-2.

According to one measurement approach, the coil driving source isapplied to the A and B terminals (corresponding to the A and B terminalsof FIG. 5(A)), i.e. the low-frequency (LF) coil supply source may beconnected to A and B terminals. On the other hand, the means for sensingincluding the low amplitude high frequency (HF) supply is connected at C(corresponding to the center tap C of FIG. 5(A)) and D terminals (with atendency to cancel each other with respect to the generation ofelectromagnetic force). In particular, by compensating the positivephase angle of an inductive impedance (e.g. L1, L2) with the negativephase angle of a capacitive impedance when put in the opposite arm (e.g.Z4, Z3) under the condition that the circuit is at resonance (i.e. nocurrent flowing between C and D), the unknown inductance (e.g. L1, L2)then can be obtained in terms of this capacitance. In another example,both LF- and HF-supply components may also be provided via A and Bterminals, and C and D terminals may be applied for sensing only.

According to another approach for measuring the varying coil inductance,the measurement AC signal (V_(CD_)ac) is applied between the C and Dterminals. The resulting AC signal between terminals A and B (V_(AB_)ac)is zero if the following relation is met: (XL2+R2)/Z4=(XL1+R1)/Z3. Theremaining V_(AB_)ac might be used to evaluate the relation between L1and L2. Z4 and Z3 might be realized as inductance equivalent (andprobably tunable) circuitry, for example a Gyrator as shown in FIG.5(D). As such, resonance is not required in this approach.

FIG. 5(C) schematically illustrates a measurement circuitry fordifferential-position sensing as illustrated in the actuation system 500according to embodiments of the disclosure. The setup of FIG. 5(C)applies the concept of the circuitry of FIG. 5(B) and may be regarded asan implementation embodiment. With the assistance of switches S1, S2, S3and S4, a bridge circuitry can be realized to measure the inductance ofL1 and L2. For example, when S1 and S4 have on-state and S2 and S3 haveoff-state, a current flows through L1 and L2, and a first measurementvalue can be obtained at the output end of the switch S7. On thecontrary, when S2 and S3 have on-state and S1 and S4 have off-state, acurrent in opposite direction flows through L1 and L2, and a measurementsignal can be obtained at the output end of the switch S6. Switch S5represents a Sample-and-Hold unit and determines the sample instance.Subsequently, the output voltage Vout (which is a differential voltage)is measured to determine the overall inductance change ΔL. When ΔL isknown, the position of the coil 503 can be obtained accordingly. SwitchS5 and amplifier X1 operate as sample and hold element for a subsequentanalog-to-digital converter (ADC).

In this respect, FIG. 5(C) shows a differential pre-amplifier to an ADC.The difference between L1 and L2 is evaluated. The input switches i.e.the pair S1/S4 as well as the pair S2/S3, are synchronized with theoutput switches S6 and S7. An offset related to ground is eliminated bythis arrangement. The differential output voltage Vout is then processedby e.g. an ADC. In this specific embodiment, the AC signal is applied atFIG. 5(B) terminals A and B, and terminal C is sensed via the sample andhold element.

FIG. 5(A)-(C) illustrate elements of measurement systems that measure arelative change in inductance for differential position sensing, i.e.using a differential approach of two inductors. However, FIG. 5(A)-(C)can also be applied to a measurement system for a single inductor(coil). In order to realize a single coil inductance measurement system,one of the inductors in FIG. 5(C) may be a Gyrator, i.e. the referenceelement may be preferably realized by a Gyrator, which is an equivalentelectronic implementation of an inductor. FIG. 5(D) schematicallyillustrates an example circuitry of a Gyrator. By modulation of thecapacitor C1 and/or the resistor R1, the value of the inductor(inductance), including a series resistance, can be modulated.

It may be noted, before starting the measurement of the VCM inductor,all current flow may be stopped and no remaining current may be in theVCM motor. That is, all the energy stored in the inductors is dischargedbefore measuring the inductors (inductance). To speed up this process,the driving terminals of the VCM have to be brought in a state thatallows increased discharge voltage via these terminals that the storedenergy can be quickly discharged. E.g. the terminals might be switchedto a high ohmic state and the energy might be discharged via theprotection diodes to the supply rails. Since the electronic speed ismuch faster than the mechanical one, the VCM remains in the givenposition during this measuring time. In real implementations, theinductor (i.e. the coil) may be the moving part and the iron may bestationary or the iron may be the moving part and the inductor may bestationary. As mentioned above, the value of the inductance changes withthe movement. For the motor, the polarity of the inductor may be keptthe same.

Similar to the actuation system 300, the position measuring unit of theactuation system 500 measures the inductance of the electric coil 503(i.e. the differential inductance ΔL of the partial coils 503-1 and503-2) and determines the position of the electric coil 503 based on thedetermined differential inductance to provide the position feedbacksignal. The position feedback signal may thus contain information of theposition of the coil 503. The actuation system 500 further includes acontrol unit (not shown in FIG. 5) for closed loop control of theposition of the optical system based on a target position and theposition feedback signal. Also, the control unit can be used forgenerating a drive signal for the electric coil 503. The coil 503 issupplied with current for its movement by the drive signal. Thus,position sensing can be achieved via the differential-position-dependingmodulation of VCM coil inductance.

The arrangement of FIG. 5(A) is based on the basic arrangement of FIG.3. That is, one can also realize the differential position sensing byadding another electric coil to the arrangement of the embodiment FIG.3. In such case, the voice coil motor further includes another electriccoil, and the position measuring unit measures a differential inductancebetween the electric coil (corresponding to 503-1) and the anotherelectric coil (corresponding to 503-2) and determines a relativeposition between the electric coil and the ferromagnetic element basedon the determined differential inductance.

As such, by means of differential measurement of the relative positiondepending on the inductance related to the individual coil sections, theactual/current relative coil position to the stationary VCM elements(e.g. the magnet 502, the ferromagnetic element 506, etc.) can beidentified. Since the partial coils 503-1(L1) and 503-2 (L2) can be moreor less overlapped by the stationary ferromagnetic material (e.g. softiron) during the coil movement, their value (inductance) changesdifferentially, and this inductance difference allows the identificationof the relative position of the coil.

Due to the differential measurement approach, residual speed of the coil(under homogeneous permanent magnetic field) should not impact theresult of the position evaluation as long as the coil is dischargedprior to measurement. Also, a discharge of the inductors prior to theevaluation may not be necessary as far as a residual speed of the coilcan be excluded, because the residual speed causes a DC offset (that canbe filtered out though). This is because a change in differentialinductance under a load current leads to a differential voltage at thecoil terminals.

It is noted that the VCM actuator described in embodiments FIG. 3 andFIG. 5(A) is not a spring-preloaded actuator. However, similararrangements may be applicable, without limitations, to aspring-preloaded actuator. A spring-preloaded actuator usually does notrequire the position sensing means as described herein, it may becombined with the disclosed principles. Furthermore, although theabove-described embodiments refer the coil of VCM as the moving part,the moving parts and the stationary parts of the above-describedembodiments are exchangeable, i.e. they can be exchanged withoutchanging the general system functionality.

FIG. 6(A) schematically illustrates an arrangement of an actuationsystem for an optical system with position sensing according toembodiments of the disclosure. The actuation system 600 comprises avoice coil motor (VCM) for actuating the optical system and may bereferred as a VCM actuator. The voice coil motor includes at least onemagnet 602 (which can be, e.g. a permanent magnet) and an electric coil603. Similar to the actuation system 300, 500, the actuation system 600also includes a control unit (not shown in FIG. 6(A)) for closed loopcontrol of the position of the optical system based on a target positionand a position feedback signal. The control unit can also be used forgenerating a drive signal for the electric coil. The coil 603 issupplied with current for its movement by the drive signal. Due to thecurrent applied to the coil 603, the coil can move in the magnetic fieldof the magnet 602 along the direction as indicated by the arrow 604. Theactuation system 600 also includes a position-measuring unit (not shownin FIG. 6(A)) for measuring the position of the electric coil andproviding the position feedback signal. According to the embodiment, theactuation system 600 further comprises an excitation unit (not shown)for generating a high frequency AC excitation signal that issuperimposed on the drive signal. In particular, the position-measuringunit measures the Back-EMF of the coil caused by the excitation signal,and determines the position of the electric coil based on the measuredBack-EMF.

In this embodiment, it is assumed that the relative position (e.g. theoverlap) between the moveable coil 603 and the static permanent magneticfield caused by the magnet 602 changes with the excursion of the coil(together with the lens barrel) arrangement. In contrast to foregoingdiscussed embodiments of FIG. 3 and FIG. 5, the position measurement isbased on a change in the back-electromotive-force (Back-EMF) voltage. Ingeneral, the Back-EMF depends on the (induced) magnetic field inside thecoil, i.e. the change rate of the magnetic flux enclosed by the coil. Inorder to generate the Back-EMF, the coil 603 keeps moving during themeasurement, which changes the magnetic flux (e.g. the static permanentmagnetic field caused by the magnet 602) inside the coil 603. Asmentioned above, the coil 603 is supplied with DC current for itsmovement. To measure the Back-EMF, another coil current component forposition measurement is applied AC-wise with subsequently changingpolarities. For example, the coil current may comprise a DC componentand an AC component, and the DC component may result in a correspondingresidual coil speed while the AC component modulates the coil speed.Besides, the Back-EMF may be favorably measured at the AC component zerocrossings.

FIG. 8 graphically illustrates variation of the Back-EMF and the coilcurrent over time. The magnitude of the Back-EMF depends on the overlap(OL) between the coil 603 and the magnet 602 (i.e. permanent magneticfield). Furthermore, the Back-EMF depends on the coil speed. Forexample, lines 801 and 804 represent the Back-EMF variation for anoverlap value of 1, and lines 802 and 803 represent the Back-EMFvariation for an overlap value of 0.5. The AC component of the coilcurrent shown in the lower part of FIG. 8 has an acceleration region 81with duration T_drv and a retardation/deceleration region 82 withduration T_drv during which a positive and negative AC currentcomponent, respectively, is applied to the coil, causing coilalternating coil acceleration and retardation, thereby correspondingvariations in Back-EMF. FIG. 8 shows the coil current as the drivingforce. In case that the impedance of the coil is dominated by itsresistance, the driving voltage would be proportional to the coilcurrent. Driving a current would otherwise have the advantage that it isnot impacted by the Back-EMF. FIG. 8 shows further the impact of aresidual coil speed on a Back-EMF offset voltage (zero offset) exemplaryfor 2 idealized overlap (between coil and permanent magnet) cases(OL=0.5 and OL=1).

Lines 801, 802 and lines 803, 804 in FIG. 8 have different offsetscaused by different residual speeds which may depend on the DC componentof the coil current applied to the coil 603 (and/or the related drivinghistory). On the other hand, the AC component of the coil currentapplied to the coil 603 results in variations in the Back-EMF, so thatthe Back-EMF, starting from an offset value, increases linearly up to amaximum value (assuming a constant acceleration of the coil due to thecoil current AC component) and then decreases linearly down to theoffset value, which reveals first an acceleration behavior for thepositive slope and then an deceleration behavior for the negative slope.The difference between the maximum Back-EMF and the offset value isindicated by delta_v (805, 806) in FIG. 8. One can observe that thedifference in the Back-EMF (delta_v) depends on the overlap (OL) value,e.g. increases along with the overlap value. Thus, the difference in theBack-EMF 805, 806 indicates the overlap between the coil 603 and themagnet 602, while the Back-EMF offset indicates (with knowledge of theoverlap) the residual speed.

As the VCM does not include a spring, there exists ideally no intrinsicmechanic resonance. As such, the Back-EMF (which is proportional to thecoil speed) should show its maxima (and minima) at each of the drivingcurrent zero crossings, i.e. at the area 83 between acceleration area 81and retardation 82 caused by the AC driving current component. Themovement of the coil during measurement may be small compared to thefull operating range of the VCM. Any ongoing (residual) single directioncoil movement during measurement may cause a related constant Back-EMFoffset. This impact on the position-dependent Back-EMF component can becanceled by evaluating the difference in Back-EMF (delta_v) insubsequent (i.e. positive and negative) coil driving current zerocrossings. With knowledge of this differential position-dependentBack-EMF component, and thereby with knowledge of the actual overlapbetween the coil and the stationary permanent magnets, the Back-EMFoffset can be used to measure the residual speed which may beadditionally taken into consideration by a controller (i.e. a controlunit) to regulate (closed loop) the coil (lens barrel) position.

Besides, the applied AC coil driving current signal for measurement may(due to the mechanic bandwidth of the VCM) not provoke a centering ofthe coil (to reach maximum overlap). As for the permanent magnetsμ_(r)=1 holds, decentering of the coil does not change the inductance ofthe coil. To minimize the energy stored within the magnetic field, forμr>1, the magnetic path in air should otherwise be minimized. Thisshould not be related to the mechanic bandwidth. The AC signal may be inprinciple applied/overlaid to a driving DC signal (for position changingof the coil). It is noted that the Back-EMF depends proportionally onthe overlap between the coil and the static magnetic field. Also, theaxial force that changes the position of the coil position dependsproportionally on this overlap and therefore also the acceleration ofthe moving coils. Accordingly, if the VCM system is excited by an ACsource for the purpose of position measurement, the Back-EMF may dependon the square of the overlap.

Although the Back-EMF may be measured in a high ohmic state (in case ofa relatively small voltage), the driving signals can be in principle(and under consideration of the Back-EMF) a voltage as well, although avoltage source typically has low impedance. This may be disadvantageousfor measuring a voltage (depending on the resistance of the Back-EMFsource). When a driving voltage signal is used, the AC source should beswitched to a high ohmic state to allow measurement of the Back-EMF. Asindicated above, the Back-EMF due to residual coil speed (i.e. theBack-EMF offset) may have a linear dependency on the overlap value,while the Back-EMF due to AC-exited coil speed (i.e. the Back-EMFdifference (delta_v)) may have a squared dependency on the overlapvalue. It is further noted that only the part of the inductor, which iscovered by the magnetic flux, can generate the Back-EMF and a motorforce. The motor may be designed to fit with the electronic design,which can drive the motor to a defined position and also to measure theposition continuously.

Accordingly, position sensing can be achieved via a VCM with variablecoil to magnetic field overlap (e.g. an overlap between the coil and themagnet). In this embodiment, no additional components are required forposition sensing (e.g. no Hall sensor and no additional ferromagneticmaterial), which simplifies the cost and complexity of the VCM system.

FIG. 6(B) schematically illustrates an equivalent circuit for positionsensing in a spring-preloaded system as illustrated in the system shownin FIG. 2(A). The low frequency mechanical resonance is due to thespring as well as the moving mass. The motor design may cause at leastone high-frequency mechanical resonance at a frequency above 20 KHz,which may change the frequency over the movement. Normally, alow-frequency mechanical resonance of around 100 Hz appears for such VCMand may be avoided. As described above, the motor may be driven by acurrent, which has a DC part and an AC part with a high frequency. Inthe circuit of FIG. 6(B), the DC part of the driving current is denotedas “Motor Driving Stimuli” and the AC part of the driving current isdenoted as “Resonance 20 kHz Stimuli”. However, the frequency of the“Resonance Stimuli” can be other values. C1 and C2 should model(together with L1 and L2 each) the electrical impact of the twomechanical resonances (e.g. 100 Hz and 20 kHz). The resonance frequencyof the mechanical system is e.g. due to nonlinearity in an affiliatedspring constant depending on the actual coil position. The driving ACstimulus is e.g., 20 kHz. It is noted that the high-frequency portionmay have no influence for the applications (e.g. the positioning of alens barrel). According to FIG. 6(B), the motor driving stimuli (thedrive signal) is the output of the regulation loop (not shown in FIG.6(B)), and the position of the coil determined based on the Back-EMFmeasurement is provided for the feedback signal.

For measuring the position of the coil, the amplitude and phase of theBack-EMF may be measured. Evaluation of the Back-EMF amplitude and phaseallows for deriving the difference between the driving AC frequency andthe (coil position depending) resonance frequency that is a measure forthe actual position of the coil.

The Back-EMF measurement may be synchronized with the driving AC stimuliand with switches S1, S2. In detail, switch S1 may be in off-state andswitch S2 may be in on-state during the measurement, so that theposition measurement using Back-EMF can be achieved at e.g. the zerocrossings of the AC stimuli and without impact of an overlain drivingsignal. After the measurement, switch S1 may be return to on-state andswitch S2 may be return to off-state. Accordingly, the phase and theamplitude of the Back-EMF can be detected. The Back-EMF frequency may bemuch higher than the driving frequency. The mechanical resonance maychange its frequency in respect of the moving position.

S1 should be opened in case that the stimulating sources are of a lowohmic type (i.e. as voltage sources) which then may require discharge ofthe coil prior to measurements. The Back-EMF will have the samefrequency than the stimuli, but amplitude and phase (compared to thestimulating signal) provide a measure for the actual coil position (asnoted above). If S1 remains closed during the measurement and probablywith the driving AC signal persisting, the evaluation of the differencein phase can—due to the relatively low Back-EMF amplitude—requirehigh-resolution instrumentation. S2 represents a sampling element (inputto e.g. an ADC).

Evaluation of the Back-EMF might require several samples to evaluate(including intra- and extrapolation) amplitude and phase. Thus,evaluation is preferably done in the vicinity of the excitation signalzero crossings (i.e. the excitation signal might be shaped in a way toallow an extended zero crossing region). Back-EMF may also be evaluatedby measuring a phase shift between excitation current and resultingvoltage. FIG. 7 schematically illustrates a flow diagram of an examplemethod 700 for operating the actuation system 600 according toembodiments of the disclosure. The method 700 comprises generating (step701) a drive signal for the electric coil. The method 700 comprisesgenerating (step 702) a high frequency excitation signal that issuperimposed on the drive signal. The method further comprises measuring(step 703) the Back-EMF of the coil caused by the excitation signal aswell as by the driving signal. Furthermore, the method comprisesdetermining (step 704) the position of the electric coil based on themeasured Back-EMF. The method also comprises providing (step 705) aposition feedback signal based on the determined position.

FIG. 9 schematically illustrates an actuation system with inherentposition sensor according to embodiments of the disclosure. Theactuation system 900 includes a VCM unit 901 and a controller 903 forrealizing the Back-EMF sensing circuitry as described e.g. in theembodiment of FIG. 6. It is noted that the controller 903 in FIG. 9 canbe used as the control unit for the embodying arrangement of FIG. 6. Asmentioned above, Back-EMF sensing may include the functionalities of (i)measurement of difference in Back-EMF at subsequent driving coil currentzero crossings to assess the overlap between the coil and the magneticfield and thereby the coil position, (ii) measurement of absoluteBack-EMF (which is optional) to assess the residual coil speed with theknowledge of the measured difference in Back-EMF and/or the measuredoverlap and (iii) means for calibration and correction, as e.g. theoverlap may not result in a linear increase in magnetic field. In someembodiments, the controller 903 may perform the position measuringfunction or comprise a position-measuring unit. In such case, thecontroller 903 may receive the measured Back-EMF directly as indicatedby the arrow 908. Alternatively, the position measurement function maybe external to controller 903.

It is noted that the Back-EMF may be preferably measured with the coil(inductance) discharged (i.e. at zero coil current) between theterminals of the coil, as illustrated in, e.g. FIG. 6(B). The input ofthe controller 903 may be provided with (apart fromcalibration/correction data) the target coil position 905 (set point)and the coil speed 906 (optionally). Actuating variable is preferablyVCM coil current. The measured Back-EMF of the VCM is sensed andprocessed (e.g. by the position measuring unit inside the controller 903or through a position measuring function performed by the controller903) to generate the instantaneous position and optional speed values,which are then considered in a closed loop control algorithm performedby the controller 903. The determined actual position value 907 may beoutput from the controller for other purposes in the VCM system. Thecontroller 903 then generates a drive signal (potentially includingoverlain AC signal) and provides the drive signal at its output (p_idrv,n_idrv) for the electric coil. The discharge switch allows dischargingthe VCM coil prior to Back-EMF measurement (e.g. at the zero crossingsof the AC signal). Finally, an enable signal en is shown in FIG. 9 toenable the controller 903.

The proposed sensing technique uses the effect of a high frequencymodulation of a resonant circuitry. It is noted that the sensingcharacteristics within the motor (VCM) may be a nonlinear function. Themotor and the employed electronic components may be adjusted fordetermining the position. The control algorithm may also includenonlinearity compensation and calibration for calibrating the totalmaximum movements. As indicated above, the moving information can be amoving distance and/or moving speed. It is further appreciated that themoving part (e.g. inductor) and the stationary part (e.g. magnet) can beexchanged.

Calibration could be done by evaluating the position of the coilcorresponding to the Back-EMF amplitude and phase and storing suchvalues (that may be interpolated in the following) within a table.

It is to be noted that the above embodiments shown in FIGS. 3, 5, 6 canalso be implemented with a linear motor. FIG. 10 schematicallyillustrates an arrangement of a linear-motor-like actuation system withposition sensing via a VCM with variable coil-magnet overlap accordingto embodiments of the disclosure. Similar to the arrangement of FIG.6(A), the VCM of the actuation system 1000 comprises a magnet 1002 (e.g.a permanent magnet) and an electric coil 1003. According to FIG. 10, thecoil 1003 has N=6 windings that are laid-out approximately flat within aplane. Due to the current applied to the coil 1003, the coil can move inthe magnetic field of the magnet 1002 along the direction as indicatedby the arrow 1004, causing a coil displacement 1005. It is appreciatedthat FIG. 10 shows a functional equivalent example of FIG. 6(A) with alinear-motor-like actuator realized instead. Thus, the functionaldescriptions given for FIG. 6 (e.g. the Back-EMF sensing, positionsensing with variable overlap between the coil and the magnetic field,etc.) are also applicable to FIG. 10. In other words, the position ofthe coil 1003 may be determined based on the overlap between the coiland the magnetic field, which can be obtained by measuring the Back-EMF.Furthermore, such a linear-motor-like actuator as illustrated in FIG. 10can also be applied to the embodiments of FIG. 3 and FIG. 5 withsimilar, equivalent realizations. It is further noted that a controllersimilar to the controller 903 in FIG. 9 can be also used for theembodying arrangements of FIG. 3 and FIG. 5 and FIG. 10, where the coilposition is determined by measuring the coil inductance.

Accordingly, the disclosure provides an actuation system/method usingthe existing parts of the VCM itself for position sensing (e.g. by usingferromagnetic material or measuring the Back-EMF for position sensing asan inherent position sensor). As such, a Hall sensor can be omitted inthe system. More specifically, the driven coil of the VCM can replacethe Hall Sensor, thereby enabling fast, efficient and accurate positioncontrol for multipurpose applications. That is, high performance VCMsystems can be achieved without using Hall sensors for positionfeedback. As such, the proposed sensing technique can make anyadditional magnets and Hall sensors for position sensing dispensable,which promises reduced system costs and increased robustness.

In particular, using the proposed position sensing technique, aVCM-based AF camera module can achieve equivalent performance withoutapplying a position sensor (Hall sensor) and superior performancecompared to a spring-preload arrangement. Thus, elements and connectionsin/between modules can be reduced, and thereby the cost, but the systemcan stay robust and can be more easily assembled.

All figures provided in this disclosure are exemplary for description ofthe addressed components, features and functionalities. They are not theresult of any system optimization that remains in the competency of theVCM/module manufactures.

It should be noted that the description and drawings merely illustratethe principles of the proposed methods and systems. Those skilled in theart will be able to implement various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope.

Furthermore, all examples and embodiment outlined in the presentdocument are principally intended expressly to be only for explanatorypurposes to help the reader in understanding the principles of theproposed methods and systems. Furthermore, all statements hereinproviding principles, aspects, and embodiments of the invention, as wellas specific examples thereof, are intended to encompass equivalentsthereof.

What is claimed is:
 1. An actuation system, comprising: a voice coilmotor comprising at least one magnet and an electric coil; aposition-measuring unit for measuring a position of the electric coiland providing a position feedback signal; and a control unit for closedloop control of a position of the electric coil based on a targetposition and the position feedback signal, and for generating a drivesignal for moving the electric coil, wherein a ferromagnetic element isarranged in proximity to the electric coil so that an inductance of theelectric coil depends on its position, and the position measuring unitmeasures the inductance of the electric coil and determines the positionof the electric coil based on the measured inductance.
 2. The system ofclaim 1, wherein the inductance of the electric coil varies based on arelative position between the electric coil and the ferromagneticelement.
 3. The system of claim 1, wherein the inductance of theelectric coil is determined based on a measured voltage and/or ameasured current.
 4. The system of claim 1, further comprising asummation unit for adding an AC signal to the drive signal for measuringthe inductance of the electric coil, wherein the AC signal has arelatively higher frequency than a frequency of the drive signal for theelectric coil.
 5. The system of claim 4, wherein the electric coil isdischarged prior to measuring the inductance of the electric coil. 6.The system of claim 1, the voice coil motor further comprising anotherelectric coil, wherein the position-measuring unit measures adifferential inductance between the electric coil and the anotherelectric coil and determines a relative position between the electriccoil and the ferromagnetic element based on the measured differentialinductance.
 7. The system of claim 6, wherein the position-measuringunit comprises a Maxwell bridge circuitry.
 8. The system of claim 6,wherein one of the electric coil and another electric coil comprises aGyrator function.
 9. An actuation system for an optical system,comprising: a voice coil motor comprising a magnet and an electric coil;a position-measuring unit for measuring a position of the electric coiland providing a position feedback signal; a control unit for closed loopcontrol of a position of the electric coil based on a target positionand the position feedback signal, and for generating a drive signal formoving the electric coil; and an excitation unit for generating a highfrequency excitation signal that is superimposed on the drive signal,wherein the position-measuring unit measures a Back-EMF of the electriccoil caused by the excitation signal, and determines the position of theelectric coil based on the measured Back-EMF.
 10. The system of claim 9,the voice coil motor further comprising at least one permanent magnetcausing a static permanent magnetic field, wherein the Back-EMF of theelectric coil varies based on a relative position between the electriccoil and the static permanent magnetic field.
 11. The system of claim 9,wherein the electric coil is discharged prior to the measured Back-EMFof the electric coil.
 12. The system of claim 11, wherein the positionmeasuring unit measures the Back-EMF of the electric coil at subsequentzero crossings of the excitation signal for the electric coil to obtaina relative position for a position determination.
 13. The system ofclaim 9, wherein the position-measuring unit measures a Back-EMF offset,wherein a residual coil speed is further obtained based on the Back-EMFoffset.
 14. The system of claim 1, further comprising a calibrating unitto obtain linear positioning.
 15. A method for actuating a voice coilmotor, the voice coil motor having a magnet and an electric coil, themethod comprising: measuring an inductance of the electric coil;determining a position of the electric coil based on the measuredinductance; providing a position feedback signal based on the determinedposition; controlling a position of the electrical system based on atarget position and the position feedback signal; and generating a drivesignal for moving the electric coil.
 16. The method of claim 15, theinductance of the electric coil depending on its position, wherein theinductance of the electric coil varies based on a relative positionbetween the electric coil and a ferromagnetic element.
 17. The method ofclaim 15, further comprising measuring a voltage and/or a currentassociated with the electric coil, wherein the inductance of theelectric coil is determined based on the measured voltage and/or themeasured current.
 18. The method of claim 15, wherein the drive signalfor the electric coil has a low frequency, further comprising adding anAC signal to the drive signal for measuring the inductance of theelectric coil, wherein the AC signal has a relatively higher frequencythan a frequency of the drive signal for the electric coil.
 19. Themethod of claim 15, further comprising discharging the electric coilprior to measuring the inductance of the electric coil.
 20. The methodof claim 15, wherein the voice coil motor further comprises anotherelectric coil, further comprising: measuring a differential inductancebetween the electric coil and another electric coil; and determining arelative position between the electric coil and a ferromagnetic elementbased on the measured differential inductance.
 21. A method foractuating a voice coil motor, the voice coil motor having a magnet andan electric coil, the method comprising: generating a drive signal formoving the electric coil; generating a high frequency excitation signalthat is superimposed on the drive signal; measuring a Back-EMF of theelectric coil caused by an excitation signal; determining a position ofthe electric coil based on the measured Back-EMF; providing a positionfeedback signal based on the determined position; and controlling aposition of the electric coil based on a target position and theposition feedback signal.
 22. The method of claim 21, wherein theBack-EMF of the electric coil varies based on a relative positionbetween the electric coil and a static permanent magnetic field causedby a permanent magnet of the voice coil motor.
 23. The method of claim21, further comprising discharging the electric coil prior to measuringthe Back-EMF of the coil.
 24. The method of claim 23, wherein a currentof the drive signal for the electric coil reaches zero when the electriccoil is discharged, further comprising measuring a difference in theBack-EMF of the coil at subsequent current zero crossings of theexcitation signal for the electric coil to obtain the relative positionfor a position determination.
 25. The method of claim 21, furthercomprising measuring a Back-EMF offset and obtaining a residual coilspeed based on the Back-EMF offset.